One motivation for the development of electrolyte-supported fuel cells having high power and high mechanical strength is their use for stationary and mobile power generation, for example, in road vehicles, in spaceflight, in combination with H2 stores (for example metal hydrides), as power stations in the megawatt range for peak loads, and as power-heating systems in the field of domestic energy.
The present invention falls within the technical field of high-temperature fuel cells. Fuel cells are electrochemical cells which convert chemical energy directly into electric energy with high efficiency. In contrast to conventional power station and turbine technology, fuel cell technology offers a high electric energy efficiency even in small plants in the range 0.1-1000 kW.
The most common chemical reaction utilized in fuel cells is the reaction between hydrogen and oxygen. Compared to low-temperature fuel cells (for example, polymer electrolyte membrane fuel cells), high-temperature fuel cells offer the advantage that, with appropriate catalytic activity of the anode, they can convert not only hydrogen but also carbon monoxide and/or methane and higher hydrocarbons, either directly or if appropriate after simple reforming using water or atmospheric oxygen, to give hydrogen- and carbon monoxide-rich fuel gas into electric energy.
In high-temperature fuel cells, separation of anode and cathode reactions which is typical of electrochemical energy converters such as batteries and fuel cells occurs by means of a ceramic membrane, namely the electrolyte, which has to be electronically insulating but conductive for hydrogen ions or oxygen ions. The present invention provides an electrolyte for a high-temperature fuel cell which is based on oxygen ion conductors, namely a solid oxide electrolyte, and an electrolyte-supported solid oxide fuel cell (SOFC=solid oxide fuel cell) which is based on this electrolyte and can be obtained by applying an anode which is electrocatalytically active for the oxidation of hydrogen and a cathode which is electrocatalytically active for the reduction of oxygen to the electrolyte which conducts oxygen ions.
A single high-temperature fuel cell produces a maximum open-circuit or zero-load voltage of about 1 volt, so that a plurality of individual cells have to be connected in series (for example, anode to cathode or bipolar), i.e., stacked, in order to produce voltages of 12 V and more which are usable in energy generation. Interconnects, also referred to as bipolar plates, are inserted between the individual cells of a fuel cell stack so as to provide, inter alia, the necessary separation of gas spaces of anode and cathode and to connect the cells firmly in an electrically conductive manner so as to make flow of current through the cell stack possible. For cost reasons, and also for reasons of operability (heating behaviour, thermal mass), the use of 0.2-0.5 mm thick, metallic bipolar plate materials is desirable. The choice of the metallic material is restricted by many requirements (including corrosion resistance, electrical conductivity of the passive layer and matching of the thermal expansion behaviour to the ceramic cell).
Known, suitable metallic materials are ferritic steels produced by melt metallurgy, such as Crofer® 22 APU from ThyssenKruppVDM, Fe—Cr alloys produced by powder metallurgy, such as the ITM alloys from Plansee, or Cr—Fe-based alloys produced by powder metallurgy, such as Ducrolloy from Plansee. The latter have the disadvantage that plate sizes as are typically required for bipolar plates cannot be produced with a thickness of less than 1.5 mm by the production technology of the prior art; the advantage is that they are corrosion resistant and mechanically strong even at 950° C. The ferritic alloys can advantageously be processed to produce thin sheets having a thickness of typically 0.3-0.5 mm; however, they have the disadvantage that they are corrosion resistant only to a maximum of 850° C.
CrFe-based alloys comprise predominantly chromium (for example 95% chromium, 5% iron for the material of Siemens/Plansee). The CrFe-based alloys can only be produced by powder metallurgy; they are shaped by pressing of the powder and cannot be machined by cutting machining nor be shaped by cold forming (for example, by bending, deep drawing, etc.) since they are too brittle.
Ferritic Fe—Cr alloys (having a maximum chromium content of 25%) can be produced by melt metallurgy like steel (for example by vacuum melting) and can therefore be rolled into sheets, machined by cutting machining and shaped by cold forming.
As an alternative to melt-metallurgical production, the ferritic Fe—Cr alloys can also be produced by powder metallurgy, such as by pressing and sintering. They then have similar properties to ferritic steels produced by melt metallurgy; however, due to the method of production they are not steels.
Fe—Cr alloys produced both by melt metallurgy and powder metallurgy which are suitable as materials for SOFC bipolar plates are characterized by a thermal expansion coefficient (TEC; all values reported below are at a reference temperature of 30° C.) in the range 11.8-12.2*10−6 K−1 at 800° C. and 10.6-10.7*10−6 K−1 at 200° C. CrFe-based alloys have a thermal expansion coefficient of 10.1*10−6 K−1 at 800° C. and of 8.9*10−6 K−1 at 200° C. Owing to the necessity of obtaining a firm bond between bipolar plate and ceramic cell, a very small difference between the thermal expansion coefficients of bipolar plate and ceramic cell is desirable.
Ceramic fuel cells can be divided roughly into the types electrolyte-, cathode-, anode- and third material-supported cells. In addition, a distinction in geometric terms is made between tubular and planar cells.
In the case of electrolyte-supported cells, the electrolyte thickness has to be at least 50-150 μm, depending on the strength of the electrolyte material and the size of the cell. Anode and cathode have, depending on structure and material, a layer thickness of 20-100 μm. In the case of cathode-supported cells, the cathode material, usually lanthanum-strontium-manganese oxide, is configured as a porous support having a thickness of about 1 mm; a thin cathode functional layer may be present on the support and a 5-15 μm thick electrolyte layer followed by a 20-100 μm thick anode layer are present thereon. In the case of anode-supported cells, the anode material, usually a cermet of nickel and (partially) stabilized zirconium(IV) oxide, is configured as a porous support having a thickness of 0.2-1.5 mm. A thin anode functional layer may be present on this and a 5-15 μm thick electrolyte layer followed by a 20-100 μm thick cathode layer are present thereon. In the case of third material-supported cells, the porous support material for the structure comprising the anode, the 5-15 μm thick electrolyte and the cathode comprises corrosion-resistant metal or inert ceramic.
The supporting material essentially determines the thermal expansion behaviour. The abbreviations of the general type “number-element-SZ” used here can be explained as follows: the number indicates the doping of the material in mol percent; the element indicates the doping element or the oxide thereof; SZ is stabilized zirconium(IV) oxide. For example, 3YSZ is a zirconium(IV) oxide doped with 3 mol % of yttrium(III) oxide; 10ScSZ is a zirconium(IV) oxide doped with 10 mol % of scandium(III) oxide; and 5YbSZ is a zirconium(IV) oxide doped with 5 mol % of ytterbium(III) oxide. Electrolytes composed of 3YSZ (zirconium(IV) oxide stabilized with 3 mol % of yttrium(III) oxide) of electrolyte-supported cells have a TEC of about 10.9*10−6 K−1 at 800° C. and of about 10.4*10−6 K−1 at 200° C. Electrolytes based on 8YSZ (zirconium(IV) oxide stabilized with 8 mol % of yttrium(III) oxide) have a TEC of about 10.1*10−6 K−1 at 800° C. and of about 8.8-9.2*10−6 K−1 at 200° C.
Anode-supported cells based on Ni/YSZ have a TEC of about 12-13*10−6 K−1. It is known that anode-supported cells harmonize with ferritic alloys, while electrolyte-supported cells based on 8YSZ (and also 10ScSZ) are, according to the prior art, used together with Cr—Fe-based alloys.
Although anode-supported cells offer very high power densities at as low as 700-800° C., they have the disadvantage that they are not mechanically stable to repeated anode-side oxidation and reduction. This behaviour forces the developer of the system to ensure that there is not an oxidizing atmosphere on the anode side, which incurs increased system costs and restricts the type of fuel gas production to steam reforming; this is generally undesirable for mobile systems and represents a restriction in the case of small stationary systems. The electrolyte-supported cells based on 8YSZ or 10ScSZ display high power densities at temperatures above 800-900° C. and can be exposed to repeated oxidation and reduction cycles on the anode side, but they have the disadvantage of a comparatively low mechanical strength which forces electrolyte thicknesses of 150 microns and more and particularly good matching of the TECs of interconnect and cell and thus the use of the thick and correspondingly expensive interconnects composed of CrFe-based alloys; otherwise, the cells break during heating and/or cooling of the fuel cell stack.
Electrolyte supported cells based on zirconium(IV) oxide doped with scandium(III) oxide (ScSZ) offer the highest power density but are disproportionately expensive in mass production because of the extremely high price of scandium(III) oxide (about 100× as high as yttrium(III) oxide, based on the molar amount), which is, inter alia, a consequence of the lack of deposits.
The combination of electrolyte-supported cells based on high-strength 3YSZ both with CrFe-based alloys (from Sulzer Hexis) and with ferritic Fe—Cr alloys (from Staxera), which are particularly inexpensive and therefore advantageous, has therefore been tested in the search for a technically simple and robust system. The differences in the TEC between interconnector and electrolyte which occur are compensated by weight or clamping forces applied to the fuel cell stack so that the high-strength, 3YSZ-based cells accommodate the mechanical stresses which occur. The disadvantage of 3YSZ-based electrolyte-supported cells is that, owing to the relatively low ionic conductivity of 3YSZ of about 2.5 S/m at 850° C., the power density of the cell is significantly lower than in the case of 8YSZ (about 8 S/m at 850° C.) or 10-11ScSZ (about 20 S/m at 850° C.) or in the case of anode-supported cells even when using relatively thin 3YSZ electrolytes (90 μm 3YSZ compared to 150 μm 8YSZ or 10 ScSZ) and larger and consequently more expensive stacks are therefore necessary to achieve a particular power.